The ability of human beings to maintain stability and balance is controlled by the vestibular system. This system provides the central nervous system with the information needed to maintain balance and stability.
FIG. 1 is a diagram showing part of the vestibular system 100. As shown, the vestibular system includes a set of ring-shaped tubes, referred to as the semicircular canals 102a-c, that are filled with the endolymph fluid. The semicircular canals are formed by a membrane called the membranous labyrinth. Each of the semicircular canals 102a-c is disposed inside a hollow bony tube (not shown in the diagram) called the bony labyrinth that extends along the contours of the semicircular canals. As further shown in FIG. 1, each semicircular canal 102a-c terminates in an enlarged balloon-shaped section called the ampulla (marked 104a-c in FIG. 1). Inside each ampulla is the cupula 106a-c, on which hair cells are embedded. Generally, as the semicircular canals 102a-c rotate due to rotational motion of a head, the endolymph fluid inside the canal will lag behind the moving canals, and thus cause the hair cells on the cupula to bend and deform. The deformed hair cells stimulate nerves attached to the hair cells, resulting in the generation of nerve signals that are sent to the central nervous system. These signals are decoded to provide the central nervous system with motion information. The three canals are mutually orthogonal and together provide information about rotation in all three spatial dimensions.
The other endorgans in the vestibular system are the otolith organs, the utricle and the saccule. These endorgans act as linear accelerometers and respond to both linear acceleration and gravity.
In response to the vestibular nerve impulses, the central nervous system experiences motion perception and controls the movement of various muscles thereby enabling the body to maintain its balance.
When some hair cells of peripheral vestibular system are damaged, but others remain viable (as often happens in situations involving bilateral vestibular hypofunction), the central nervous system of a person receives inaccurate information regarding the person's motion. As a result, the person's ability to maintain stability and balance will be compromised. Persons with improperly functioning vestibular systems may consequently experience vertigo, dizziness, and clumsiness, which may lead to collisions and spontaneous falls.
Another type of vestibular system affliction is Meniere's disease. Meniere's disease is a medical condition in which the vestibular system, for unknown reasons, suddenly begins varying the pulse-repetition frequency of the vestibular signal, even when the patient is stationary. This results in severe dizziness. Subsequently, and again for no known reason, the vestibular system begins generating a vestibular signal consistent with the person's spatial orientation, thereby ending the person's symptoms.
One way to remedy symptoms associated with ailments that result in the central nervous system receiving inaccurate motion information is to use prostheses based on electrical stimulation. Such prostheses use implanted or non-implanted transmitting electrodes to cause electrical stimulation of a target nerve (e.g., vestibular nerve ganglion cells). Such electrical stimulation results, for example, in corresponding reflexive responses in the vestibulo-ocular and the vestibulo-spinal pathways, thereby enabling the person to maintain balance and stability in response to the electrical stimulation. Alternatively, such electrodes can target nerves other than those associated with the vestibular system.
Similarly, to alleviate symptoms of Meniere's disease, electrical prostheses can be used to provide a stationary signal to the brain. This can be achieved by producing a jamming signal, through electrical stimulation, that, when combined with a non-stationary signal present on the vestibular nerve, causes the vestibular nerve to provide a stationary signal to the brain. A description of the use of electrical, mechanical, and chemical stimulation of the vestibular system to alleviate Meniere's disease symptoms is provided in U.S. patent application Ser. No. 10/738,920, entitled “Vestibular Stimulator”, filed Dec. 16, 2003, the contents of which are hereby incorporated herein by reference in their entirety.
Although useful in providing some relief from vestibular system afflictions, electrical stimulation tends to affect large nerve areas. Such stimulation, therefore, is less useful when refined or focused stimulation is sought. Moreover, electrical stimulation is generally performed using electrodes that have to be positioned proximate to the nerves that those electrode will target. Consequently, when the target nerves are the nerves of the vestibular system, the electrodes have to be surgically implanted close to those nerves. Such a surgical procedure often necessitates cutting through bones surrounding the target nerves, thus resulting in considerable collateral damage to the affected area.